The present invention relates to optoelectronic sensors, and more particularly optoelectronic liquid level sensors and detectors which provide indication of sensor component failure irrespective of environmental conditions.
Optoelectronic liquid level sensors are well known in the art, and are typically used to detect the presence or absence of a liquid or for sensing whether the level of liquid in a vessel has risen above or fallen below a predetermined depth. Optoelectronic sensors are primarily used as point type liquid detectors, although they are sometimes used conversely as point type air bubble detectors, since they are capable of distinguishing between the different media directly in contact with surface portions thereof.
Operation of such devices utilizes principles of internal reflection governed by the differences of refractive indices between an optically transparent probe portion, and the surrounding medium. The probe portion is commonly provided in the form of cylinder having an optically polished conical tip. A light beam is directed through the probe portion in the direction of the conically formed tip. The probe is made of a material having an index of refraction such that the angle at which the light impinges the surface of the conical tip from the interior of the probe is greater than the critical angle when the probe surface is exposed to air, whereby virtually all of the light is internally reflected. When exposed to liquid, the critical angle, determined as a known function of the ratio of the indices of refraction of the probe material and the surrounding media (i.e. gas or liquid) with which it is in contact, is greater than the angle of incidence, resulting in transmission of the impinging light beam passed internally through the probe outward of the conical tip, refracted into the surrounding medium. A light detector, positioned to receive the light that is internally reflected when the probe is exposed to air (or other gas with similar index of refraction), provides means for producing a measurable circuit parameter responsive to an amount of light received thereby, by which xe2x80x9cwetxe2x80x9d and xe2x80x9cdryxe2x80x9d conditions can be determined. For convenience, a light source and a light detector are placed side by side and spaced apart in a location at the circular face of the cylindrical portion of the probe, and the conical tip configured at a 90xc2x0 angle, such that the angle of incidence is approximately 45xc2x0, and the beam from the light source is reflected back to the light sensor along a return path approximately parallel wit the incident beam.
In practice, such optoelectronic sensors comprise a light emitting diode (L.E.D.) as the light source, a phototransistor (P.T.) as the light detector, and a housing to protect the operational components of the sensor and to facilitate mounting thereof. Sometimes, for convenience, the prism and housing are machined or molded as one piece from suitable material, such as transparent plastic.
The drawback of such sensors is that should the operational components, including the light source and/or the detector, fail, there is no practical way of distinguishing between such failure and the presence of liquid. This is particularly problematic in applications where it is important to know when liquid has dropped below a certain level, since the probe is ordinarily not responding to the internal light beam when immersed and will not indicate any change upon light failure, giving a continuously false reading that liquid is still present at point of measurement. Also, in applications where the sensor is not easily accessible, determination of whether the components are operational by removal and testing thereof is impractical. In such cases it is impossible to distinguish between a reading generated by presence of liquid and product failure.
It would therefore be desirable to be able to monitor the functionality of a liquid sensor without adding additional circuitry or wiring to it which would otherwise contribute to the cost of such device and decrease its statistical reliability because of higher component counts and the number of extra solder joints required. Furthermore, since these optoelectronic liquid sensors typically have integrated circuit means for producing a voltage on a single output lead representative of the condition of the sensor, it would also be desirable to permit such failure-detecting monitoring strictly by determination of a single output voltage level, or other conveniently measurable circuit parameter. Ideally, such fail-safe feature would be included in an optoelectronic sensor arrangement in a manner obviating elaborate structural configuration of the various components which might complicate manufacture or limit a range of suitable material from which they are fabricated.
Accordingly, it is an object of the invention to provide an optoelectronic liquid sensor which overcomes the drawbacks of the prior art.
It is a further object of the invention to provide an optoelectronic liquid sensor in which continuous monitoring of the functionality of the various operational components of a liquid sensor may be implemented.
It is a still further object of the invention to provide an optoelectronic liquid sensor in which failure detection of the various components may be determined by conversion of a measurable circuit parameter into an output level representative of the conditions of wet, dry and sensor failure, such as for example, production of three discrete voltage levels corresponding to each of the sensor states.
Yet another object of the invention is to provide an optoelectronic sensor which integrates a conversion circuit as part of a self-contained sensor which producing a discrete voltage signal on an output line.
It is yet another object of the invention to provide such a failure detecting feature in an optoelectronic sensor in a simple, reliable and cost effective manner.
Briefly stated, the present invention provides an optoelectronic liquid sensor providing facilitated failure detection of the various operative components contained therein. The sensor includes a light source, a light detector, and a transparent probe which functions as a primary liquid sensing prism. The transparent probe is of a suitable configuration permitting internal reflection of light transmitted therethrough from the light source, along an internally reflected primary light path for reception by the light detector, when the light-impinged interior surface of the primary prism is in air. When the light impinged surface of the probe is submersed in or exposed to liquid, light from the light source traveling along the primary light path, instead of being internally reflected, is refractively transmitted into the liquid, and therefore almost no light reaches via the primary light path. In addition to the primary light path, a secondary light path is provided in the sensor, along which a fraction of the light from the light source is analogously transmitted via internal reflection within a transparent body to the light detector. This is accomplished conveniently, for example, by a secondary prism which is isolated from ambient exposure such that it is consistently exposed to an air or gaseous environment, and which is trapezoidally configured in cross-section and optically polished to internally reflect the received light. Alternatively, such secondary light path may be provided by structural configuration of the transparent probe which functions also as the primary liquid sensing prism. In either embodiment, as long as the sensor components are operable, a portion of light transmitted by the light source and internally reflected along the secondary light path is consistently received by the light detector. The light detector is wired as part of a suitable circuit, the design of which is at least partially dependent upon the nature of the type detector, for detecting changes in the level of the particular circuit parameter altered as a result of the particular detector""s exposure to light levels, for example a change in current, and also including means for converting such parameter value into output values representative of three discrete levels corresponding to conditions of, wet, dry and sensor failure, respectively.
In accordance with these and other objects of the invention, there is provided an embodiment of a sensor in accordance with the invention which includes a light source conveniently in the form of a light emitting diode, a phototransistor light detector, and a transparent probe which functions as a primary liquid sensing prism. A secondary prism, which is trapezoidally configured in cross-section, internally reflects a portion of light output from the L.E.D. along a secondary light path to be received by the phototransistor.
According to a further feature of the invention, there is further provided another embodiment in which the primary light prism as described above is specially configured to present secondary structure enabling transference of light via internal reflection along a secondary light path within the single transparent probe.
The present invention also includes the above embodiments wherein, an entire sensor body is fashioned in one piece, and includes external structure facilitating mounting in an application. Alternatively, the components may be mounted in a separately provided housing of suitable material and construction.
In an advantageous embodiment, each of the above embodiments may be combined with a circuit integral with the sensor, such that external conversion of a circuit parameter influenced by a level of light received by the detector is obviated. Conveniently, this is provided as a printed circuit housed directly in the sensor, and encapsulated.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.